This invention relates to a digital audio system which converts a speech signal to digital data, and particularly to a muting circuit which mutes noises generated instantaneously during the process of turning off the power, or noise of null data generated by the data which is damaged by the influence of external or internal circuitry.
In a digital audio system, usually, media for recording or a radio frequency signal for broadcasting are input to the audio system and converted into digital data to be processed. Then, it is converted back into an analog signal which is an audible speech signal. There are various kinds of digital audio systems such as a digital tape recorder, a compact disc player, a pulse code modulation (PCM) processor, a digital tuner and a digital amplifier.
In a prior muting circuit in a digital audio system, as is shown in FIG. 1, an electronic switch SW is connected between an output terminal of a low frequency amplifier 10, for amplifying the converted analog speech signal and outputting it to a given next stage, and a ground terminal. The electronic switch SW operates by a muting signal from a system controller (not shown) through a line 13. Then, the output of the low frequency amplifier 10 is passed to the ground terminal through the electronic switch SW so that the noises are muted.
However, the speech signal 2a of FIG. 2 input to the low frequency amplifier 10 is output in the form of the speech signal as shown in waveform 2c and is distorted by the affection of instantaneous pop noises when the electronic switch SW is turned on at the moment t that a signal for muting shown as waveform 2b changes abruptly to a logic high state. Therefore, there is a problem such that, at the point of muting, disturbing beat noises are generated by the pop noise or a transient phenomenon of the audio system.